This invention relates to fine-grained, polycrystalline, glass-ceramic fibers produced from vitreous fibers of a basalt-type material, and particularly useful for reinforcing purposes.
It has become customary to employ asbestos fibers as a reinforcing agent in concrete structures such as pipes, tiles, panels, and the like. However, an extensive search has been underway for a suitable substitute because of the health hazards involved in handling asbestos material.
Glass fibers have been among the various alternatives under consideration. Since 1935, the glass fiber industry has become firmly established, and glass fibers are used in many widely diversified applications. One such application is the reinforcement of plastic materials used in molding structures ranging from musical instruments to automobile bodies.
Unfortunately, commercially available glass fibers have failed to provide all of the characteristics required for concrete reinforcement. In particular, the available fibers lack resistance to the alkaline environment that prevails when the common cements, such as Portland cement, are mixed with water. Glasses have been developed having a high resistance to an alkaline environment, but the difficulty in producing fibers from such glasses, and/or the intrinsic cost of such fiber production, have militated against their consideration for this purpose.
Basalt-type materials are widely available, and relatively inexpensive, natural materials. They are generally classified in mineralogy as basic volcanic rocks wherein the essential constituents are the minerals feldspar, pyroxene, and magnetite, with or without olivine, and a black basalt glass. It has been recognized that they are resistant to alkaline attack, a characteristic that makes them of particular interest in connection with alkaline environments such as are encountered in concrete work. Basalt materials are also easily melted and drawn as fibers, a further fact that makes them of interest as potential concrete reinforcement fibers. A detailed description of these materials, their physical and chemical characteristics, and their commercial history, is set forth in U.S. Pat. No. 3,557,575, granted Jan. 26, 1971 to G. H. Beall and H. L. Rittler.
That patent discloses the production of glass-ceramic articles from basalt-type materials. In accordance with its method, a basalt type batch, containing at least 5% Fe.sub.2 O.sub.3, is melted; the melt is cooled below its transformation range and a glass article shaped therefrom; the glass is heat treated in the range of 640.degree.-675.degree. C. to form magnetite nuclei; and the nucleated glass is further heat treated to cause crystallization on the nuclei. Thus, the patent teaches that a magnetite crystal phase can be otained by heat treatment in the range of 675.degree.-800.degree. C., while heat treatment at a higher temperature in the range of 850.degree.-1000.degree. C. forms a clinopyroxene-magnetite mixed phase in which the former predominates. Heat treatment times of 0.5 to 4 hours are suggested.
It is observed that the glass-ceramic bodies have substantially better resistance to attack by either acid or alkaline solutions than do corresponding glass bodies. This resistance is ascribed to a residual glass phase rich in alumina and silica. The patent warns that large plagioclase spherulites tend to form if heat treating temperatures above 1000.degree. C. are employed.
Recently, it was proposed, in U.S. Pat. No. 4,008,094, granted Feb. 15, 1977 to the inventors Beall and Rittler, that the resistance of basalt-type glass fibers to alkaline attack be improved by including 1 to 15% zirconia (ZrO.sub.2) in the composition of the basalt melt from which the fibers were drawn. The patent further suggests that the glass fibers may be partially crystallized by internal crystallization to develop magnetic properties. Heat treatments ranging in time from 1 to 4 hours, and in temperature from 650.degree. to 900.degree. C., are proposed to develop a crystal phase composed of magnetite (Fe.sub.3 O.sub.4), ulvospinel (Fe.sub.2 TiO.sub.2), or solid solutions of these crystal forms. Necessarily, such crystal phases represent only a minor degree of crystallization since the total content of Fe.sub.2 O.sub.3 plus TiO.sub.2 in the glass is no more than about 10 to 15% by weight. The patent also notes that temperatures much in excess of 900.degree. C. ought to be avoided since coarse-grained crystals of pyroxenes can develop.
It has been recognized that glass-ceramic materials tend to have greater inherent mechanical strength than their glass precursors since a dominant crystal structure is developed that is homogeneous and relatively fine-grained. In contrast, large, coarse crystals in a body normally tend to weaken the material and render it brittle, and hence are avoided.